In thermocouple circuits it is necessary, for certain measurements, to utilise two identical thermocouples and, whilst keeping one at a reference temperature, to use the other to sense the temperature of an environment to be investigated. Under laboratory conditions, the temperature of the first thermocouple is usually maintained at ice point (0.degree. C) by use of an ice bath, or at some higher fixed temperature by means of a temperature-controlled oven, and the first thermocouple is usually known as the reference or "cold junction". The other thermocouple is inserted in the environment under investigation and is known as the "sensing junction" or "hot junction". The known conventional methods for maintaining the first temperature-controlled junction at a constant temperature are satisfactory for use under laboratory conditions, but are impractical in terms of weight, size, cost, power consumption, maintenance, warm-up time, and ice replacement or contamination, in many industrial applications, and especially in aircraft and missile applications.